The present invention relates to a golf ball including a core and a cover coating the core, and more particularly, to a golf ball with a degradation-preventable core.
A golf ball includes a core formed of a material having good repulsive elasticity as a main component, and a cover protecting the core and having dimples directly involved in flight aerodynamically. The golf ball may include a single core, or may include a dual core, a triple core, etc., to attain diverse performance. Also, the golf ball has a single outermost cover having dimples thereon, however, the golf ball may have various structures, for example, including a double-layered cover in which an inner cover is formed inside an outer cover having dimples, a triple-layered cover having three covers, a quadruple-layered cover having four covers, as occasionally demands.
The core (including the dual core and the triple core, and hereinafter will be referred to as ‘core’) plays a key role in various golf balls. The core plays a key role in obtaining the driving distance of a golf ball by generating a strong repulsive elasticity when hitting by a golf club. For this reason, as a material for the core, cis-1,4-polybutadiene rubber (hereinafter will be referred to as ‘polybutadiene rubber’) having the greatest repulsive elasticity among polymer materials is widely used, and sometimes, other rubber components may be partially mixed therewith. The core is manufactured by using α,β-ethylenically unsaturated carboxylic acid, an ester of the unsaturated carboxylic acid or a metallic salt of the unsaturated carboxylic acid and an additive such as a filler for cross-linking and curing with a base rubber. The core occupies the largest volume in the golf ball and is a very important part. The core is generally distinguished from the cover protecting the core from breaking, involved in flight characteristic, and having a relatively small volume. The cover is formed by using a polymer material having a lower elasticity than the material used for the core, however having an appropriate elasticity and strong physical properties, such as an ionomer resin, polyester, a polyethylester copolymer, a thermosetting or thermoplastic polyurethane, a polyamide resin, a thermoplastic rubber, and the like.
As described above, to manufacture the core which plays the key role in the golf ball, a polybutadiene rubber or a mixture of the polybutadiene rubber and another rubber added thereto, is mixed with α,β-ethylenically unsaturated carboxylic acid, an ester of the unsaturated carboxylic acid or a metallic salt of the unsaturated carboxylic acid, which enables the polybutadiene rubber to be cross-linked and cured, and an additive such as a filler, and an organic peroxide is then added as a reaction initiator to a resultant mixture. The resultant mixture is cut into an appropriate size, and then molded, cross-linked and cured in a heat compression molding machine or a rubber injection molding machine provided with a mold having a certain shape at about 165° C. and above for about 15 minutes or more, thereby forming the core. The polybutadiene rubber generally used for making a core which is light as a specific gravity of about 0.91 g/cm3, and other rubber miscible with the polybutadiene rubber is also considerably light, a heavy filler is necessary to be additionally added to control the weight of the core. When the specific gravity of the filler is low and an added amount of the filler is thus increased, the strength of the core obtained may be degraded, and the core may be easily broken when the golf ball is hit by a golf club. Of course, the decrease of the strength due to the excessive amount of the filler may be prevented by using a filler having relatively high specific gravity to some extent. However, an inhomogeneous mixture may be easily formed even due to a trivial mistake while performing a mixing process, a difference of the specific gravity be caused in the mixture, which makes it difficult to obtain a product having uniform quality. In addition, the cross-linking process is performed at an extremely high temperature using the peroxide as a reaction initiator during a molding process. Thus, the cross-linked and cured rubber becomes weak by repeated stress and is unable to maintain the strength for the desired number of hits. Particularly, the rubber becomes weaker by the hit when the core is the dual core or the triple core than the single core.
As described above, a problem caused by the molding process performed at a high temperature is degradation of the rubber which inevitably occurs as long as the core of a golf ball is produced by using the thermosetting rubber as a base material. The degradation may be reduced by various methods, however the degradation may be inevitable to some extent in a reaction system in which a side chain is developing during the cross-linking and curing reaction. Further, in the case of high stress is applied in one direction to the core during molding in one direction, crack or burst may be frequently generated in a direction perpendicular to the applied stress direction when an external impact is applied to the core taken out from the mold after finishing the molding process. In this case, the core may not be using in a product. If such a core does not found during an intermediate inspection that is applied to a finished product and released in a market, the finished product may be easily broken by a very weak hit. This leads to a user's claim.
Generally, the rubber molecules are degraded by the main chains broken due to deteriorate chemically or physically, that caused the physical properties to be lowered, or the surface of the rubber to be sticky and weakened. During chemical bonding, cross-linking may occur at the partially broken site by the attack of oxygen to an α-hydrogen site or a double bond site, and flexibility and elongation of the rubber molecule may be further decreased. Generally, natural or synthetic rubbers virtually include some chemically unsaturated bonds liable to be attacked by the oxygen, therefore, when a metal compound is included in a mixture, the degradation may proceed more rapidly around a metal compound part, because of the strong catalytic reaction of the metal compound. However, the reaction is mostly focused on the main chain during cross-linking and curing of the polybutadiene rubber having a long linear bonding, and the degradation may be considerably decreased. The molding at the high temperature is preferably finished after conducting the reaction under appropriate conditions for a certain time period. If the reaction is performed for a prolonged time period exceeding an appropriate time period, an excessively high temperature heat may be applied to the cross-liked and cured core, which accelerating the degradation of the rubber. Thus, it is necessary that the reaction time period be accurately kept and the cooling process be performed. Moreover, strong static electricity generated by the polybutadiene rubber when mixing a mixture which may allow oxygen, present in the air directly above the rubber molecules that converted into ozone. When the mixture is subjected to the cross-linking reaction with the ozone mixed therein, linear main chain of the rubber may be partially broken first to accelerate the degradation. Furthermore, later, the ozone remaining during molding may also react with an organic peroxide which is added in the mixture as a reaction initiator, and may trigger a violent reaction throughout the core when a temperature reaches a certain temperature and more. In this case, carboxyl radical may not participate in the cross-linking reaction, and may partially cleave the main chain of the rubber molecule and unsaturated carboxylic acid at an undesired site. This may accelerate degradation having certain directivity afterward. The degradation due to the ozone may not be restrained by using a general antioxidant, and the degradation of a product may not be prevented. So, a golf ball having a core preventing the above-described degradation and a strong repulsive elasticity when hitting by a golf club that has been disclosed.